Turbo car gear ratios for drag strip, I need some advice!

S

Speedfreak426

Forum Member
I need some decision making guidance here guys...


Some background info:
I have a 1977 Toyota Celica that I swapped a 92 turbo supra engine and R154 manual transmission into.

Rear axle is out of a '68 big block Dart, a 8.75" with a 3.91 gear.

I built myself a Megasquirt system for the car, all assembly and tuning is and will be done by my lonesome.
Currently I only have a rough tune, about 1 hour into making the volumetric efficiency map, and I just made an educated guess on timing, because I was running out of dyno time and I just wanted to make some shakedown runs at Milan.

Well, I ran a 11.51 @ 122mph, not too bad for not being tuned yet! It should have a lot left in it once I get the tune all set, and up the boost some more. It is set up for 2 step launch control, but I didn't use it on these runs. Running around 16psi right now, and I plan on 20+ once I get the roll cage in. So it will be somewhere in the 10 second range. (and 30+MPG, I just thought i'd throw that in there :naughty1: )


Well, at the strip, I ended up breaking the Mopar differential, which is the last thing I thought I would break!
It is a cone style limited slip, and I busted off one tooth off of the one side gear, luckily I was still able to drive it home!

This is pretty much a strictly street car, but I do like to take it to Milan once or twice a year.

More background info / cliffnotes:
-I am crossing the finish line at 6300ish rpm's (6250 oem redline) in 4th gear @ 122-123mph
-Safe to spin up to 6500.
-current 3.91 rear gear
-235-60-15 MT DR's (26.1" dia)
-R154 Transmission gear ratios:
First Gear: 3.250:1
Second Gear: 1.955:1
Third Gear: 1.310:1
Fourth Gear: 1.00:1
Fifth Gear: 0.753:1
-broke the diff, looking to upgrade and change gear.

some pics of what I have:
IMG_2896.jpg

IMG_3325.jpg


So, finally on to my questions:

Main question- at the drag strip I don't want to use an overdrive gear correct? I assume the max I should be using is 4th. Since I am already at redline in 4th I am under the impression that I should change the diff ratio since I plan to go faster next year. There are not a lot of options for the 8.75" out there under 4.30:1
Since this is a turbo car I would think keeping the car in boost as long as possible would help my et's, or I could be wrong and it will slow me down.... what do you guys with some turbo car knowledge think? any suggestions?
My options are- 1: buy another 3.91 gearset and shift to fifth. 2: 3.73 gearset will put me at 6250 rpm at 130mph in 4th, I don't know how fast I will get this thing going in the future, but I only want to upgrade once.
3: 3.55 will be 136 @ 6250 4: I can buy a 4:30+ gear and get it up into 5th gear, but I don't know what would happen with spooling the turbo and the time lost from an extra shift.

Does anyone have any real world experience with turbo cars and changing ratios at the strip? any input is greatly apreciated!

I need to buy a new carrier, looking at the Eaton Truetrac torsen diff and a few others.
Has anyone had good experience with the truetrac? It seems like it would be pretty awesome for a street car.
 
You dont want to use an overdrive gear in hard accels. You definately want to put a taller gear in the rear. I would go to atleast a 3.55, but that depends on the power the car is making.. I run 3.31's in my Mustang, and alot of guys seem to like the 3.27's
 
I don't have anything to add to this, but I just wanted to say that despite being an import that is a neat project you have.
 
Turbo's like the lower gears as it loads them up better. You don't need tall gears with a turbo. And it might let you cross the line a little lower in your rpm range. I have 3:42 in my car and they work great with my set-up. Finally killed the rear in my car last season so may be upgrading to an 8.8 conversion which uses a 3:50 gear. So I should be pretty close to where I was before.
 
Birdie2000 said:
I don't have anything to add to this, but I just wanted to say that despite being an import that is a neat project you have.
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Thank you! It sure does suprise a lot of people!
93 saturn sl2 said:
Swap in a 8.8 if possible and go with 3.27s unless they make a similar ratio for your axle.
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Meh, 8.75 will be plenty strong, it's a 4 link setup and took quite a bit of work to swap it over, not doing that again. trying to save some $$ here. 3.27 is not available :/
Hugger Z said:
Turbo's like the lower gears as it loads them up better. You don't need tall gears with a turbo. And it might let you cross the line a little lower in your rpm range. I have 3:42 in my car and they work great with my set-up. Finally killed the rear in my car last season so may be upgrading to an 8.8 conversion which uses a 3:50 gear. So I should be pretty close to where I was before.
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Thank you for your insight. I will try to find a 3.55 gear for my axle, but it is not looking good so far. Dodge used 3 different castings for this rear axle, and I am having trouble finding anything lower than a 3.73 :/

I am just on a pretty tight budget, so I can't afford to buy a whole new case, I'll see what I can find tho.
 
Speedfreak426 said:
I will try to find a 3.55 gear for my axle, but it is not looking good so far. Dodge used 3 different castings for this rear axle, and I am having trouble finding anything lower than a 3.73 :/
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That's what makes it difficult. Give DTS a call, they might be able to help. We have a 3.55 in our 8.75.
 
Im not expert, but it sounds to me that your 3.91 is the perfect gear for your setup right now. If your crossing the line in 4th gear at 6300 RPMS and your safe to 6500... id say your pretty much spot on what the car can go. I mean you dont want to be in OD when your going cross the line. Maybe you could go with your other option of a 3.73 and see what that does??
 
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